1. Field of the Invention
The present invention relates to a semiconductor laser and a method for producing the same, more particularly relates to a semiconductor laser constituted by a III-V group compound semiconductor such as GaAs and a method for producing the same.
2. Description of the Related Art
A semiconductor laser using an AlGaInP quaternary mix-crystal semiconductor on a GaAs substrate produces visual light of the 600 nm band and is used as a light source of an optical pick-up in a digital versatile disc (DVD) or other optical disc apparatus or of a laser pointer etc.
In such a semiconductor device, optical damage (hereinafter also referred to as COD (catastrophic optical damage)) occurs when the laser is operated while raising the output of the laser beam. It has been known that this is caused by the fact that the interfacial level in the regions near the end faces of a resonator of the semiconductor laser causes the bandgap at those regions near the end faces to become small.
For example, a method for realizing a higher output of a semiconductor laser by providing a window structure having a larger bandgap (energy width) than an oscillation wavelength of the semiconductor laser in the region near the optical output end faces of a resonator having a multiple quantum well structure and preventing the optical damage at this optical output end has been developed.
For example, Japanese Unexamined Patent Publication (Kokai) No. 7-58402 discloses a method of diffusing Zn in the regions near the end faces of the resonator of the semiconductor laser to scramble the multiple quantum well structure and thereby to expand the bandgap.
Further, for example Japanese Unexamined Patent Publication (Kokai) No. 10-200190 discloses a method in which holes are formed by introducing nitrogen into the regions near the end faces of the resonator of the semiconductor laser as an impurity by ion implantation, the holes are made to diffuse in the active region by annealing, and the quantum well structure is made a mixed-crystal one by the interaction between a well layer and barrier layers so as to expand the bandgap.
Summarizing the problems to be solved by the invention, in a semiconductor laser diffusing zinc in the regions near the end faces of the resonator, however, since the zinc easily diffuses, there is the problem that the zinc introduced in the active layer having the window structure diffuses to other layers, the light is absorbed by the diffused Zn, and therefore the performance of the laser is lowered.
Further, there is the problem that the leakage current of the regions near the end faces of the resonator increases due to the zinc and an oxidation reaction is accelerated by energy released at the time of non-light emitting recombination of carriers due to that leakage current, so the laser end face changes and the reliability is lowered.
On the other hand, in a semiconductor laser diffusing vacancies by introducing nitrogen into the regions near the end faces of the resonator by ion implantation, since the impurity atoms (N) do not diffuse up to the active region, the problems described do not occur, but there is a problem that since the ion radius of nitrogen is large, it is hard to implant the nitrogen and therefore an implantation energy of about 150 keV is required, the crystal is damaged at the time of this ion implantation, and therefore the reliability of the laser is lowered.
Further, there exists another problem in that, in the annealing after the ion implantation, a high temperature treatment of 850xc2x0 C. is required. This temperature is near the crystal growth temperature of the semiconductor layer, so deterioration of the crystal layer occurs along with the annealing.
An object of the present invention is to provide a semiconductor laser enabling a higher output without lowering the performance of the laser such as its reliability and a method for producing the same.
To achieve this object, according to a first aspect of the present invention, there is provided a Fabry-Perot type semiconductor laser having, successively grown on a substrate, a first cladding layer of a first conductivity type, an active layer having at least one quantum well layer and at least two barrier layers, and a second cladding layer of a second conductivity type, a pair of facing end faces of said active layer constituting a resonator, wherein an impurity and vacancies due to the impurity are diffused in at least one region near the end faces of said active layer, and the quantum well layer and the barrier layers constituting said active layer are made mixed crystals.
In the semiconductor laser of the present invention, preferably boron is introduced as said impurity in at least one region near the end faces of said active layer. Further, preferably, in the regions near the pair of facing end faces of said active layer, the quantum well layer and the barrier layers constituting said active layer are made mixed-crystals. Further preferably, boron is introduced as said impurity in regions near the pair of facing end faces of said active layer.
In the semiconductor laser of the present invention, since the impurity and the vacancies due to the impurity are diffused in at least one region near the end faces of the active layer having at least one quantum well layer and at least two barrier layers and the quantum well layer and the barrier layers constituting the active layer are made mixed-crystals, it is possible to expand the bandgap of the regions near the end faces of the resonator of the semiconductor laser.
By using for example boron or another impurity, the impurity and the vacancies due to the impurity can be diffused, so it has become possible to use the effect of the impurity and the effect of the vacancies together. Namely, boron ions can be implanted into the semiconductor layer with a low implantation energy, and the diffusion of the impurity and the diffusion of the vacancies can be achieved at a low annealing temperature. Further, boron has a slower diffusion rate than that of zinc and another impurity used up to the present, so the diffusion of the impurity into the active layer can be adequately controlled.
For this reason, the problem of damage to the crystal layer at the time of ion implantation and at the time of a high annealing temperature, which has been a problem up to the present, or problems such as absorption of the light due to the impurity and an increase to the leakage current are suppressed, the bandgap of the regions near the end faces of the resonator of the semiconductor laser is expanded, and a higher output of the semiconductor laser is thereby enabled.
Further, to achieve the object, according to a second aspect of the present invention, there is provided a method of producing a Fabry-Perot type semiconductor laser in which a pair of facing end surfaces of an active layer constitute a resonator, comprising a step of forming a first cladding layer of a first conductivity type on a substrate, a step of forming an active layer having at least one quantum well layer and at least two barrier layers above said first cladding layer, a setup of forming a second cladding layer of a second conductivity type above said active layer, and a step of diffusing an impurity and vacancies due to the impurity in said active layer in predetermined regions acting as end faces constituting said resonator and making the quantum well layer and the barrier layers constituting said active layer mixed-crystals.
In the method for producing a semiconductor laser of the present invention, the first cladding layer of the first conductivity type is formed on the substrate, the active layer having at least one quantum well layer and at least two barrier layers is formed above the first cladding layer, and the second cladding layer of the second conductivity type is formed above the active layer. Next, the impurity and the vacancies due to the impurity are diffused in the active layer in predetermined regions acting as end faces constituting the resonator of the semiconductor layer and the quantum well layer and the barrier layers constituting the active layer are made mixed-crystals.
In the method for producing a semiconductor laser of the present invention, preferably the step of diffusing the impurity and the vacancies due to the impurity in said active layer in the predetermined regions acting as end faces constituting said resonator includes a step of introducing said impurity in predetermined regions acting as the end faces constituting said resonator and a heat treatment step for diffusing said impurity and the vacancies due to the impurity in said active layer in said predetermined regions.
In the method for producing the semiconductor laser of the present invention, preferably boron is used as said impurity. Further, said heat treatment step is preferably carried out at a temperature not more than the crystal growth temperature of the semiconductor layer and more preferably is carried out at a temperature of 750xc2x0 C. or less.
According to the method for producing a semiconductor laser of the present invention, since boron or another impurity and the vacancies due to the impurity are diffused in the active layer in the predetermined regions acting as end faces constituting the resonator of the semiconductor laser and the quantum well layer and the barrier layers constituting the active layer are made mixed-crystals, it is possible to implant the boron ions into the semiconductor layer with a low implantation energy and it is possible to effect the diffusion of the impurity and the diffusion of vacancies at a low annealing temperature not more than the crystal growth temperature of the semiconductor layer, for example 750xc2x0 C. or less. Further, since boron has a slower diffusion rate than that of zinc and other impurities used up to the present, the diffusion of the impurity into the active layer can be adequately controlled.
Accordingly, the problem such as the damage to the crystal layer at the time of ion implantation and at the time of a high annealing temperature or problems such as the absorption of the light due to the impurity and the increase of the leakage current are suppressed, the bandgap of the regions near the end faces of the resonator of the semiconductor laser is expanded, and therefore a higher output of the semiconductor laser is enabled.